The oil demand of a hydraulically actuated automatic transmission is the sum of several fractions. A first fraction of the operating medium to be provided by an oil supply system is needed for pressurizing the transmission shift elements required for torque transmission, such as clutches. In a first operating condition in which no shifts to change the gear transmission ratio are carried out, a small volume flow to compensate for leakage losses in the clutches acted upon with a clutch pressure for the corresponding transmission ratios is needed. The operating medium, which can even be other than oil, is therefore under a clutch pressure. For a second operating condition, also referred to as transmission shifting in which the transmission ratio is changed, a larger volume flow is required for a short time for filling the clutches that have to be pressurized with the clutch pressure in the new transmission ratio step.
A further fraction of the operating medium is needed for lubricating and cooling the transmission components, but the pressure required for this is substantially lower than for pressurizing the transmission shift elements. Thus, in a hydraulic system as described there are at least two different hydraulic circuits with different pressure levels, each of which has to be supplied with a particular volume flow.
Known hydraulic systems of automatic transmissions are supplied with the operating medium required for actuating the transmission by means of a transmission pump in the form of a displacement pump. The operating medium is delivered by the transmission pump under a certain pressure with a volume flow sufficient to actuate the clutches in the desired manner. In addition, certain transmission components have to be lubricated and/or cooled at low pressure.
In this context the displacement pump has a fixed displacement volume, i.e. one that cannot be varied. The displacement volume is the volume of operating medium that can be geometrically displaced per pump revolution. Since the displacement volume cannot be varied, the volume flow delivered by the transmission pump increases proportionally with the rotation speed of an internal combustion engine driving the transmission pump mechanically.
The displacement volume is designed in accordance with the minimum volume flow demand at given speeds, these speeds in known applications being in the lower part of the internal combustion engine rotation speed range. In addition, the peak demand for the short time when clutches have to be filled during transmission shifts must be covered. Consequently, in the first driving condition in which it is only necessary to top up the leakage quantity, a volume flow at the clutch pressure level provides a large excess. Owing to the proportional relation between the volume flow delivered and the rotation speed of the transmission pump, this means that on passing through the speed range from low to high speeds, there is a further volume flow increase after reaching the minimum demand of the transmission, giving a volume flow excess which is not needed for actuating the transmission shift elements or for cooling or lubrication. Precisely at high rotation speeds this results in large power loss, since the superfluous volume flow is discharged, via a valve, into a transmission housing under ambient pressure, with reduction of the pressure previously produced by means of mechanical power. The energy stored in that pressure is then transformed into heat, which disadvantageously raises the temperature of the operating medium. Moreover, the volume flow sprayed out into the transmission housing causes severe foaming of the operating medium, in particular in the case of oil, and this results in undesired pressure fluctuations, noise, and damage to the pump.
To adapt to the demands of the hydraulic system, the use of displacement pumps with variable displacement volumes is known. The displacement volume varies between a minimum and a maximum value, such that the minimum displacement volume can have values down to zero. Until a minimum demand that serves as a design starting point is reached at a certain speed, the maximum displacement volume is maintained. If the demand of the transmission is covered, the displacement volume can be made still smaller, for example with increasing speed, so that the volume flow remains theoretically constant, although any other variation of the volume flow with speed can also be produced. For peak volume flow demands, for example when filling the clutches during a shift operation, the displacement volume can be briefly increased, but in practice the reaction time until it is adjusted and ultimately the volume flow increase cannot take place spontaneously enough. To be able to reach the design point located in the lower speed range, the displacement volume must be set to its maximum value and the volume flow then increases proportionally with the speed. Only when that speed is reached at which the volume flow demand is covered, can the displacement volume of the transmission pump be reduced. Since in practice this speed is close to the speed that exists at the top end of the consumption-relevant driving cycle, the advantages of the adjustable displacement pumps have no effect on fuel consumption. Disadvantageously, owing to its structural configuration the efficiency of an adjustable displacement pump in the operating range with maximum displacement volume is lower than that of a non-adjustable pump. Furthermore, the structural complexity and thus the cost of an adjustable transmission pump are greater than those of a displacement pump with a fixed displacement volume.
To decouple the volume flow delivered by a displacement pump and the variable rotation speed of an internal combustion engine, electrically powered transmission pumps are known. In this case a displacement pump with a fixed displacement volume is no longer driven by the internal combustion engine but by means of an electric motor, whereby the volume flow can be adjusted independently of the speed of the internal combustion engine. Here, the pressure or volume flow adjustment takes place by varying the speed of the electric motor. The disadvantages in this case are that the volume flow increase is insufficiently rapid to cover a peak demand at the moment of transmission shifting, and the efficiency is low because of the twofold energy conversion when electrical energy is produced by a generator and mechanical energy is transferred from the electric motor to the transmission pump.
In DE 10 2004 025 764 A1 a hydraulic system for the oil supply of a multi-step automatic transmission for motor vehicles is described, in which two pumps are arranged in the transmission. In this case one pump supplies the transmission shift elements and a second pump operating at a lower pressure level is responsible for lubricating and cooling the transmission. To be able to cover the greater oil demand due to filling of the transmission shift elements during a shift operation, in a first version, the pressure in the second pump is raised above the pressure level of the first pump by means of an adjustable pressure-limiting valve so that the volume flow of the second pump flows to the transmission shift elements through a one-way valve in addition to the volume flow of the first pump. In another version, at the moment of the shift operation the two volume flows are combined by means of a multi-channel valve in order to fill the transmission shift elements. Since both pumps are mechanically driven there is still a disadvantageous dependence of the volume flow on the pump rotation speed and thus a high power loss in the upper speed range, with the negative consequences already described.
DE 197 50 675 C1 shows an oil supply system for a transmission with two pumps, such that a first pump is driven by an electric motor and a second pump by an internal combustion engine or a transmission input shaft. The first, electrically driven pump ensures the basic supply to the clutches, i.e. in a first operating condition of the transmission it produces in the clutches the pressure required for torque transmission and compensates for the leakage volume flow. The pressure here is relatively high and the volume flow small. The basic supply for lubrication and cooling comes from the second pump, whose rotational speed is proportional to that of the internal combustion engine, and in this case compared with the first pump a substantially larger volume flow is delivered at comparatively low pressure. The pressure side of the first pump and the pressure side of the second pump are connected to a valve device whose principle is illustrated. If, now, when the transmission is shifted there is for a short time an elevated volume flow demand for filling the new clutches to be pressurized, then the pressure side of the second pump is connected by means of the valve device to the pressure side of the first pump. In what follows, the hydraulic system in which the pump conveys the operating medium will be referred to in general as the pressure side. In this way the volume flow of the second pump is added completely or partially to that of the first pump, so that the elevated clutch demand can be covered. For the case when, at low speeds and due to the proportional speed dependence of the volume flow of the second pump, the supply for lubrication and cooling is no longer sufficient, the valve device can be shifted so as to connect the first pump to the lubrication and cooling circuit, whereby it reinforces the second pump. The structure and operating mode of the valve device are not shown. A disadvantage in this case is that the displacement volumes of the two pumps are chosen such that only when acting together can they cover the peak demand at the time of the gear shift. Thus, if the electrically driven pump fails it is no longer possible to shift the transmission.